Here’s the relevant chart from Cryosphere Today. I downloaded it last night, and added the crude red circle round the interesting bit (click the image to see the current version of CT’s graph — it updates daily).

It covers the entire satellite record, and shows the anomaly — that is, the difference between the actual ice area for a given day and the average ice area for that day over the entire period. Looking at the anomaly should remove the annual cycle, because that’s accounted for in the average that forms the baseline. Nevertheless, from 2007 onwards it looks very much like the anomaly is itself showing an annual cycle.

Here’s a close up. The first big drop occurs in 2007, the year of the record summer minimum area, but it’s there again in 2008 and 2009 — and shows every sign of happening again this year. If you look closely, you’ll see something just as interesting as the cycle itself. The maximum anomaly occurs after the summer sea ice minimum. This is obvious on CT’s graph of area over last two years. The red line at the bottom is the anomaly, and you can see that the maximum anomaly occurs after the ice area has started to regrow — which means that the freeze-up has begun later than the 30-year average. However, once the ice starts refreezing, the anomaly decreases rapidly and gets up towards the zero line over winter. So what’s going on? Why is the late summer anomaly so much greater than the winter anomaly, and why have we only started seeing the cycle in the last few years? Here’s my stab at an explanation…

Consider the geography of the Arctic sea ice. You can divide it very roughly into two regions — the ice in the Arctic basin, that is the ocean around the pole, north of Canada, Alaska, Siberia, Svalbard and Greenland, and the sea ice that forms outside the Arctic basin (Barents Sea, Bering Strait, Kamchatka, etc). Every winter the Arctic basin freezes up. That happens every year without fail (so far). The winter anomaly therefore depends on the amount of sea ice that grows outside the central Arctic. But all that ice melts long before we get anywhere near the late-summer ice minimum — in other words, it’s irrelevant to prospects for the summer.

Summer anomalies are determined by the ice melt in the central Arctic. Up to 2007, the summer melt mainly took place around the Canadian and Siberian shores, and in the Barents Sea. That’s why the Northwest and Northeast Passages were/are a tricky proposition — they depend on the fringes of the polar sea ice cap melting. In 2007, however, ice over a large chunk of the central Arctic basin melted away, setting a new record minimum by a 25% margin. That delayed the start of the freeze-up significantly, and so the anomaly increased to more than 2.5m km2 below average. But this is the central Arctic we’re talking about, and still a very chilly place, so even though the freeze-up was delayed, the sea ice regrew as autumn turned into winter and covered the entire basin.

The emergence of an annual cycle in the anomaly therefore occurs because there’s now more variability in summer area/extent than there is winter area/extent, and because there is, in absolute terms, more central Arctic ice to lose. 2007′s record melt triggered the appearance of the cycle. And as and when we hit new record minima, we’re likely to see even bigger swings in the cycle as long as the entire central Arctic freezes up every winter.

[This is not news to real ice scientists, by the way, because model runs show similar sea ice behaviour as the ice declines -- though it's being seen rather sooner than modelling suggested. This post at Primaklima at Scienceblogs.de shows some examples. H/t to Georg himself in comments at µWatts]

There is another tantalising hint to be discerned in the new cycle. In each of the last three winters, the anomaly has approached zero — and sceptics have been keen to trumpet the ice as being back to normal. As we’ve seen, this is because of the growth of sea ice outside the Arctic basin, and so it is irrelevant to the state of the ice that will melt the following summer. The slightly reduced winter anomalies (compared to the four or five years prior to 2007, though not earlier) might be related to changes in weather patterns outside the central Arctic (which has been warmer than average throughout recent winters, while parts of Siberia have been colder), or perhaps to reductions in salinity caused by greater run-off of fresh water (making it easier to freeze) — but that’s highly speculative.

It’s interesting to note that the annual area anomaly cycle is to a certain extent mirrored in the PIOMAS ice volume data. Here’s the last few years snipped from their latest chart. There was a record volume anomaly in summer 2007, as you might expect, followed by a recovery over winter as the ice cover regrew. Another minimum occurred in summer 2008, followed by a smaller regrowth, and then another drop to a record minimum in summer 2009. Not much new ice since, though, and in the last couple of months the volume anomaly has plummeted. They’ve had to add another section to the bottom of the chart since I last posted it here…

Talk of betting on the summer minimum is underway at the mustelid’s place, and I am on record there as suggesting that this summer will see a greater melt than the last couple of years. A new record? Perhaps, but I’m not betting on it. Meanwhile, the SEARCH forecasting exercise has posted its first “pre-release” ice forecast: Adrienne Tivy, a post-doctoral fellow at the International Arctic Research Center (IARC) has developed a statistical model that projects a 4.539m km2 Sept average, “below normal” but way above the last three years. Still, when you look down on the top of the world (or bottom, from my perspective) from the vantage of NASA’s Terra and Aqua satellites, the eyeballs in the sky (there is life beyond the pooliverse!) make the sea ice look very broken up and mobile. Click on the little image at the top of the post (which shows open water at the western entrances to the NW Passage yesterday) to see the latest Arctic mosaic, and make up your own mind…

The usualsuspects have been making much of the fact that over the last few weeks Arctic sea ice extent (NSIDC daily graph here) has been bumping around the 30 year average for this time of year. John Cook at Skeptical Scienceposted on the subject last weekend, making the important point that what matters most is not extent or area, but the total volume of ice that’s present — and that’s showing no signs of “recovery”. John’s post is well worth reading, but it set me off on a very interesting trawl through the references he provided — and drew my attention to a most useful graph of ice volume and trend. It also pointed me to research that suggests the Arctic could be effectively ice-free in summer within ten years — possibly as soon as 2013.

Here’s the graph that caught my eye. It’s produced by the Polar Science Center at the the University of Washington in the USA, and is regularly updated — this was generated on April 17th.

It shows the sea ice volume anomaly and its trend, as calculated by the Pan-Arctic Ice Ocean Modelling and Assimilation System (PIOMAS) developed Dr J Zhang and his colleagues at the PSC. It’s important to note that this is the anomaly that’s being shown — the difference between the ice volume on a given day and the average for that day over the full 79-09 satellite record (in other words, the seasonal cycle is removed). The blue line is the trend (- 3,300 km3 per decade) and the grey bars show one and two standard deviations from that trend. Two things are I think obvious: the ice volume has been steadily declining over the last 30 years, and the rate of loss looks to have increased markedly over the last ten years. We’ll come back to that point…

Now lets compare that graph to the sea ice area anomaly as calculated by the Polar Research Group at the University of Illinois, from their Cryosphere Today site.

I grabbed that graph a day or so ago, from here. It updates daily, so the end point will have changed, but the history won’t. Again, we’re looking at the difference from average for any given day in the record, with the seasonal cycle removed. There’s been a steady decline over the last 30 years, and a noticeable steepening over the last ten. So what’s the difference between area and volume? The thickness of the ice, of course. Estimates of extent or area give us only 2D information — but the amount of ice floating on the Arctic Ocean is the area times the thickness.

Now focus on the most recent three years on both graphs. You’ll recall that Arctic sea ice recorded a new record low summer minimum in 2007. Looking at the area anomaly, you can see that clearly as the lowest spike on the graph — the minimum was over 2.5 million km2 below average. The anomaly then gets smaller as the winter freeze-up continues, bigger again in summer, and so on. What’s striking is that the amplitude of the “cycle” between low and high anomalies has become much larger since that 2007 record melt. Why that might happen is an interesting question (for another day, perhaps).

The PIOMAS data provide a different perspective. The 2007 summer low is obvious on the graph, but it is no longer the lowest point. That was reached in 2009. In Feb-Mar 2009 the Arctic sea ice volume was 11,900 km3 — below the average minimum September volume over 79-09, and the current volume is only slightly above that. In other words, the sea ice hasn’t “recovered” by any stretch of the imagination — it’s two standard deviations below the trend, which is already showing decline of 3,300 km3 per decade. The PIOMAS numbers reflect the loss of thicker multi-year ice in recent years, replaced by thinner one and two year old ice.

What happens if you look at the recent sea ice volume trend and project it into the future? Here’s a graph from a presentation [pdf] by the US Naval Postgraduate School’s Prof Wieslaw Maslowski, given at the ARCUS State of the Arctic conference in Miami last month (anyone interested in Arctic research will find a treasure trove of material at the conference web site):

This shows the October-November ice volume over the last 30 years and five different estimates of the current rate of decline in those months. The most recent data (from Ron Kwok’s team at NASA) is consistent with a rate of decline of about 1,000 km3 per year. The green line on the graph shows the volume of ice left in the Arctic when there’s a bit clinging to the Canadian archipelago and Greenland (see the little map above). Maslowski therefore projects an “ice-free fall by 2016 (±3yrs uncertainty)”. Not having been present at his talk, I can’t tell you exactly how he put that in words, but the graph and quote come from his slides. Maslowski is well known for his aggressive projections for summer sea ice loss, but this is the first time I’ve seen his argument quantified.

I’m still mulling over the implications (not least for sea ice bets). On one level, it’s confirmation of a worrying projection — an ice-free Arctic Ocean in late summer within the next ten years. The consequences of that, for both the climate of the northern hemisphere and the geopolitics of the Arctic are huge. On the other hand, there are undoubtedly arguments as to why this might not happen — at least, not so soon. A run of cool summers could allow the ice volume to rebuild as it did in the early 1980s. First year ice could become second year ice, then third year and so on. That seems to be within the range of natural variation in the PIOMAS numbers, but it might only postpone the inevitable by what — five or ten years?

Here’s the context: the IPCC’s fourth report put Arctic summer sea ice loss out into the second half of the century. Current volume numbers are consistent with loss within ten years. Warmer Arctic autumns mean snowier northern hemisphere winters and significant changes in weather patterns. The sea ice is not recovering — instead it shows that rapid climate change is happening here and now. And that’s not alarmist, it’s truly alarming.

[Beach Boys: apologies for the appalling pun, but I'm running out of relevant song titles for ice posts. All suggestions gratefully received.]

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